1. Field of the Invention
The present invention relates to a color ink-jet printer capable of ejecting ink droplets of different colors.
2. Discussion of Related Art
U.S. Pat. No. 6,416,149 B2 (in particular, FIGS. 4–6, and col. 1, lines 46–53 and col. 5, lines 43–50) corresponding to JP-2001-301206A discloses an ink-jet printer operable such that at least one droplet of an ink each having a predetermined volume is ejected from each nozzle, so as to form a dot of ink on a recording medium in the form of a paper sheet, such that where a plurality of ink droplets are ejected from the nozzle, the ink droplets overlap each other so as to form one ink dot. Thus, a desired gray-scale value can be established at each picture element of an image corresponding to each ink dot to be formed on the paper sheet according to image data (printing data), by suitably selecting one of three different total volume values (large, medium and small values) of the above-indicated at least one ink droplet, for each of the ink dots, so that each ink dot has the corresponding one of three different sizes or diameters which are determined by the respective three different total volume values.
Publication JP-4-364961A of Japanese Patent Application (in particular, paragraphs [0020] and [0021]) discloses a color ink-jet printer capable of ejecting droplets of inks of different colors which have different compositions so that the different-color inks are given different rates of fixing (different drying speeds) on a recording medium, to prevent undesirable color mixing due to bleeding of the different-color ink droplets at their boundaries. Namely, the color mixing at the boundaries can be prevented since one of the adjacent two dots of the inks of the different compositions is dried at a higher rate than the other dot.
If the technique disclosed in the above-identified U.S. Patent is applied to a color ink-jet printer having a plurality of rows of nozzles that are arranged to eject droplets of inks of respective different colors (e.g., yellow [Y], magenta [M], cyan [C] and black [B]), the same number of the ink droplets corresponding to one ink dot are ejected for each of the different colors, that is, the total volume of the ink droplets corresponding to one ink dot is the same for all of the different colors, when the gray-scale value at a picture element in question is the same for all of the four colors. Where the different-color inks have the respective different compositions as disclosed in the above-identified Japanese Publication, it is possible to reduce a risk of deterioration of quality of a printed image due to the color mixing at the adjacent ink dots of different colors. However, the different-color inks of different compositions have respective different drying speeds, and the viscosity of the ink having a relatively high drying speed may be excessively increased at the meniscus surface of the ink remaining in a given nozzle, due to evaporation of an aqueous component of the ink at the meniscus surface, which takes place if the ejection of the ink droplets from that nozzle is absent for a relatively long time. In this case, the nozzle may suffer from so-called “plugging” due to increased viscosity of the ink at the meniscus surface, leading to a failure to subsequently eject the ink droplets from that nozzle. In this respect, a color ink-jet printer is required to operate without not only the color mixing at the adjacent ink dots of different colors, but also the plugging of the nozzles due to the different compositions of the different-color inks.